Suitability of soil microbial parameters as indicators of heavy metal pollution

Several microbial parameters (microbial biomass, respiration, dehydrogenase, phosphatase, sulphatase, glucosidase, protease and urease activities) were measured in soils from five sites located in urban green areas close to roads differing in traffic density. Our aims were to evaluate the suitability of such parameters as field biomarkers of stress induced by heavy metal pollution, and to compare results obtained by single microbial parameters with results given by an index expressing the average microbial (AME) response of the microbial community. Data showed that all parameters were significantly reduced in the sites characterized by the highest load of metals in soil. Dehydrogenase, sulphatase, glucosidase activities and respiration, declined exponentially with increasing metal concentration, whereas phosphatase activity and AME decreased following a sigmoidal type relationship. In contrast, protease, urease and microbial biomass were not significantly correlated with soil metal concentration. Microbial parameters differed both in sensitivity to critical metal concentrations and in the rate of decline at increasing metal loads in soil. Due to the complex interplay of chemical, physical and biological factors which influence microbial activities and biomass, the proposed index (AME) appeared more suitable than single microbial parameters for a biomonitoring study of this type.     

Biochar as a strategy to sequester carbon and increase yield in durum wheat

Carbon sequestration in agricultural soils is a climate change mitigation option since most of cultivated soils are depleted of soil organic carbon and far from saturation. The management practices, most frequently suggested to increase soil organic carbon content have variable effects depending on pedo-climatic conditions and have to be applied for a long time periods to maintain their sink capacity. Biochar (BC), a carbon rich product obtained through carbonization of biomass, can be used for carbon sequestration by applying large amounts of carbon very resistant to decomposition. The BC remains into soil for a long time and there is evidence that the BC stores atmospheric carbon from centennial, to millennial timescales. However most of the agronomic studies on BC application have been made in tropical and sub-tropical climates, while there is a substantial lack of studies at mid-latitudes and in temperate climates. This paper presents the results on an investigation of large volume application of BC (30 and 60 t ha^?1) on durum wheat in the Mediterranean climate condition, showing the viability of BC application for carbon sequestration on this crop. BC application also has positive effects up to 30% on biomass production and yield, with no differences in grain nitrogen content. Moreover no significant differences between the two BC treatments were detected, suggesting that even very high BC application rates promote plant growth and are, certainly, not detrimental. The effect of the biochar on durum wheat was sustained for two consecutive seasons when BC application was not repeated in the second year.     

Effect of cycloheximide on N2O and NO3– production in a forest and an agricultural soil

The present work aims at evaluating the effect of cycloheximide at concentrations of between 0.5 and 5mgg^–1 on N₂O and NO₃ ^– production in two slightly alkaline soils, sampled from deciduous woodland and arable cultivation. In the first experiment, peptone was used as the “inducing substrate” for heterotrophic activity, and soil was incubated with cycloheximide (at different concentrations) and/or acetylene (1mll^–1) to block induced eukaryotic protein synthesis and ammonia monooxygenase activity, respectively. Peptone addition stimulated N₂O and NO₃ ^– production significantly in woodland soil, whereas arable soil showed no significant N₂O emissions and low NO₃ ^– production. Low cycloheximide concentrations drastically reduced N₂O emissions in woodland soil, suggesting a potential role of fungi in N₂O emissions. However, acetylene was equally effective in blocking N₂O emissions and part of NO₃ ^– production, so that a possible role of ammonia monooxygenase in an organic-inorganic pathway of N nitrification in fungal metabolism can be hypothesized. A second experiment was carried out on the woodland soil to check if low cycloheximide concentrations had non-target biocidal effects on soil microorganisms. Attention was focused on the range of concentrations which had reduced N₂O emission in the woodland soil. The results suggested that at concentrations of cycloheximide between 0.5 and 2mgg^–1 any biocidal effect on microbial biomass was negligible in the first 48h; therefore only selective inhibition of protein synthesis could be expected. The whole nitrifier population seemed to be particularly sensitive to cycloheximide concentrations higher than 2.5mgg^–1. Received: 4 July 1997     

Impact of fire on fungal abundance and microbial efficiency in C assimilation and mineralisation in a Mediterranean maquis soil

The present study investigates the impact of fire (low and high severity) on soil fungal abundance and microbial efficiency in C assimilation and mineralisation in a Mediterranean maquis area of Southern Italy over 2 years after fire. In burned and control soils total and active fungal mycelium, microbial biomass C, percentage of microbial biomass C present as fungal C, metabolic quotient (qCO₂) and coefficient of endogenous mineralisation (CEM) were assayed together with several chemical properties of soil (i.e. pH and contents of organic C, total and mineral N, available K, Mg, Mn and water). Fire significantly decreased the fungal mycelium, whereas it stimulated microbial growth probably through the enhancement of bacterial growth because of the increase in organic C and nutrient contents in burned plots. This shift in microbial community composition might explain the observed reduction in soil microbial efficiency of C assimilation (high qCO₂) and the increase in C mineralisation rate (CEM) in the first 84 days after fire. Therefore, fire might increase CO₂ input to the atmosphere not only during combustion phase but also in the post-fire period.     

Cycloheximide inhibition of peptone-induced nitrate production across a soil moisture gradient

The antibiotic block technique is used to distinguish between fungal and bacterial induced activity. In the present study, the antibiotic inhibition of peptone-induced NO₃^– production was tested across a soil moisture gradient. Soil was incubated at 60, 80, 90 and 100% water-filled pore space (WFPS) and as a water slurry. Peptone was used as the substrate and cycloheximide and C₂H₂ (0.1% v/v) were added to inhibit fungal and autotrophic nitrification, respectively, the latter being considered mainly of bacterial origin. At all moisture contents is more than 80% of NO₃^– production was due to autotrophic nitrification. At increasing water contents the percentage of NO₃^– production inhibited by C₂H₂ increased, whereas the percentage inhibited by cycloheximide decreased from 26.4% at 60% WFPS to 4.6% in the water slurry, suggesting a different sensitivity of bacterial and fungal nitrification to soil moisture. Although no direct evidence of an alteration in the fungal population was produced in this experiment, data proved that water content influences the result of the test and hence care should be taken when comparing data using different test conditions.     

Fungi-to-bacteria ratio in soils of European Russia

The selective inhibition technique by specific antibiotics (streptomycin, cycloheximide) applied to substrate-induced respiration (SIR) measurement was used to test the relative contribution of fungi to bacteria (F/B ratio) to the overall microflora-induced activity in soils of European Russia. Investigated soils covered a wide climatic transect and different ecosystem types including managed vs. natural ecosystems. Before direct comparison among sites, the antibiotic inhibition technique was optimized for soil characteristics. Once the optimal concentration was set, the combined effect of the two antibiotics resulted in average 60% inhibition of SIR. The analyzed sites (in total 47) including various biomes (tundra, middle taiga, southern taiga, subtaiga, dark coniferous forests outside the boreal region, steppe, mountain forests and arable sites), were characterized by a wide range of soil pH_w (3.95–7.95), soil organic carbon (0.69–24.08%), soil microbial biomass carbon (149–5028 µg C g^?1 soil) and soil basal respiration (0.24–8.28 µg CO₂-C g^?1 soil h^?1). In all the analyzed sites, a predominance of fungal over bacteria activity was observed with F/B ratios always higher than one (4.9 on average). Natural sites were characterized by higher F/B ratios (on average 5.6) compared to agricultural ones (on average 3.5).     

Responses of nitrous oxide, dinitrogen and carbon dioxide production and oxygen consumption to temperature in forest and agricultural light-textured soils determined by model experiment

 The experiment, carried out on a forest and arable light-textured soil, was designed to study the temperature response of autotrophic and heterotrophic N₂O production and investigate how the N₂O flux relates to soil respiration and O₂ consumption. Although N₂O production seemed to be stimulated by a temperature increase in both soils, the relationship between production rate and temperature was different in the two soils. This seemed to depend on the different contribution of nitrification and denitrification to the overall N₂O flux. In the forest soil, almost all N₂O was derived from nitrification, and its production rate rose linearly from 2  °C to 40  °C. A stronger effect of temperature on N₂O production was observed in the arable soil, apparently as a result of an incremental contribution of denitrification to the overall N₂O flux with rising temperature. The soil respiration rate increased exponentially with temperature and was significantly correlated with N₂O production. O₂ consumption stimulated denitrification in both soils. In the arable soil, N₂O and N₂ production increased exponentially with decreasing O₂ concentration, though N₂O was the main gas produced at any temperature. In the forest soil, only the N₂ flux was related exponentially to O₂ consumption and it outweighed the rate of N₂O production only at >34  °C. Thus, it appears that in the forest soil, where nitrification was the main source of N₂O, temperature affected the N₂O flux less dramatically than in the arable soil, where a temperature increase strongly stimulated N₂O production by enhancing favourable conditions for denitrification. Received: 26 August 1998     

Role of root exudates on the sorption of arsenate by ferrihydrite

Iron oxy‐hydroxides in soil are known to have a large affinity for arsenate (As(V)) inorganic species. At the soil–root interface such mineral components are embedded by mucilaginous material that is secreted from continuously growing root cap cells. In order to determine the role of plant mucilages in As(V) sorption by iron oxy‐hydroxides, we layered a calcium (Ca)‐polygalacturonate network (CaPGA) on to amorphous iron (Fe) (III) hydroxide (ferrihydrite, Fh) particles. The scanning electron micrographs of the CaPGA network coating the ferrihydrite (Fh–CaPGA) show a regular structure with a honeycomb‐like pattern where interlacing fibrils form a porous system. The FT‐IR spectra of Fh–CaPGA suggest that CaPGA fibrils are retained by the surface Fe(III) nuclei of Fh through electrostatic interactions. The sorption experiments carried out at pH 4.3 and 5.8 indicated a smaller amount of As(V) sorbed by Fh–CaPGA than by Fh alone, being less after 3 and 24 hours of reaction by about 70 and 30%, respectively. The sorption of As(V) by Fh was also studied in the presence of caffeic acid (CAF), an important root exudate. Simultaneous sorption kinetics show that As(V) sorption by Fh is almost independent of CAF concentration, indicating a greater affinity of arsenate ions towards the Fh surfaces. However, the amount of As(V) sorbed by the Fh coated by CaPGA, in the presence of 0.25, 0.5 and 1.0 mm CAF, is markedly smaller by about 20, 27 and 40%, respectively, than that found in the As(V)–CAF‐Fh ternary systems. This is caused mainly by redox reactions involving CAF and the surface Fe(III) nuclei of Fh leading to the formation of CAF oxidation products which prevent As(V) sorption.